The present invention relaters to silicone rubber particles coated with a silicone resin and a method for the preparation thereof. More particularly, the invention relates to silicone resin-coated fine silicone rubber particles exhibiting excellent flowability and dispersibility in and compatibility with various kinds of matrix materials to give a composite material imparted with markedly decreased internal stress and greatly improved surface lubricity as well as to a method for the preparation of such silicone rubber particles.
It is known in the prior art that fine particles can be prepared from several silicone materials, i.e. organopolysiloxanes, including cured silicone rubbers and poly(organosilsesquioxane) resins. These silicone particles are useful as an additive ingredient in various kinds of matrix materials such as rubbers, plastics, coating compositions or paints, printing inks, waxes, cosmetic compositions and the like with an object to impart the matrix material with improved heat and cold resistance, weatherability, water-repellency, surface lubiricity and mold releasability as well as to decrease the internal stress of shaped articles. Fine particles of a silicone rubber are particularly preferred when decrease in the internal stress of shaped articles is desired.
Various methods have been proposed for the preparation of fine particles of a silicone rubber, for example, in U.S. Pat. No. 3,843,601, Japanese Patent Kokai No. 59-96122, No. 62-243621, No. 62-257939, No. 62-270660, No. 63-77942, No. 63-312324 and elsewhere. One of the problems in the silicone rubber fine particles prepared by these prior art methods is that the particles are poorly flowable to cause inconveniences in handling and they can hardly be dispersed in various matrix materials with full uniformity because the particles strongly agglomerate in addition to the relatively low effect of internal stress relaxation of shaped articles due to their poor compatibility with the matrix material.
A method is proposed in Japanese Patent Kokai No. 4-348143 and No. 5-179144, according to which fine particles of a silicone rubber are compounded with fine inorganic particles to improve the flowability of the particles with decreased agglomeration but no solution is obtained thereby for the problem of poor compatibility of silicone rubber particles with matrix materials.
Further, several proposals and attempts have been made with an object to improve the compatibility of silicone rubber particles with various matrix materials including admixture of the silicone rubber with an epoxy compound disclosed in Japanese Patent Kokai No. 64-56735 and admixture of the silicone rubber with an alkoxy silane compound disclosed in Japanese Patent Kokai No. 64-700558. The improvement in the compatibility of silicone rubber particles obtained by these methods, however, is still not high enough rather with some adverse influences on the flowability and agglomeration of the silicone rubber particles.
Apart from silicone rubber particles, Japanese Patent Kokai No. 5-170909 discloses a method for the preparation of an aqueous dispersion of which the dispersed phase consists of fine particles of a fluorocarbon resin containing an organopolysiloxane formed by in situ hydrolysis-condensation of a hydrolyzable organosilane compound in the resin particles. This method, however, is not suggestive for the improvement of silicone rubber particles relative to the flowability and agglomeration.